page_title: Transition state theory
section_title: Quasi-equilibrium assumption
hierarchical_section_title: Transition state theory / Justification for the Eyring equation / Quasi-equilibrium assumption
caption_attribution_description: English: Potential-energy diagram
context_page_description: Transition state theory explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium between reactants and activated transition state complexes.
  TST is used primarily to understand qualitatively how chemical reactions take place. TST has been less successful in its original goal of calculating absolute reaction rate constants because the calculation of absolute reaction rates requires precise knowledge of potential energy surfaces, but it has been successful in calculating the standard enthalpy of activation, the standard entropy of activation, and the standard Gibbs energy of activation for a particular reaction if its rate constant has been experimentally determined.
  This theory was developed simultaneously in 1935 by Henry Eyring, then at Princeton University, and by Meredith Gwynne Evans and Michael Polanyi of the University of Manchester. TST is also referred to as "activated-complex theory," "absolute-rate theory," and "theory of absolute reaction rates."
  Before the development of TST, the Arrhenius rate law was widely used to determine energies for the reaction barrier.
context_section_description: Quasi-equilibrium is different from classical chemical equilibrium, but can be described using the same thermodynamic treatment. Consider the reaction below
  where complete equilibrium is achieved between all the species in the system including activated complexes, [AB]‡ .  Using statistical mechanics, concentration of [AB]‡ can be calculated in terms of the concentration of A and B.
  TST assumes that even when the reactants and products are not in equilibrium with each other, the activated complexes are in quasi-equilibrium with the reactants. As illustrated in Figure 2, at any instant of time, there are a few activated complexes, and some were reactant molecules in the immediate past, which are designated [ABₗ]‡ (since they are moving from left to right). The remainder of them were product molecules in the immediate past ([ABᵣ]‡).  Since the system is in complete equilibrium, the concentrations of [ABₗ] ‡ and [ABᵣ]‡ are equal, so that each concentration is equal to one-half of the total concentration of activated complexes:
  In TST, it is assumed that the flux of activated complexes in the two directions are independent of each other. That is, if all the product molecules were suddenly removed from the reaction system, the flow of [ABᵣ]‡ stops, but there is still a flow from left to right. Hence, to be technically correct, the reactants are in equilibrium only with [ABₗ]‡, the activated complexes that were reactants in the immediate past.
caption: Figure 2: Potential energy diagram
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